"Gelled" solid polymer electrolytes comprise a polymer having, incorporated therein, a plasticizer containing organic solvents and a lithium salt. The plasticizer is trapped in the three-dimensional structure of the polymer. The resulting substance forms a single phase.
Gelled electrolytes based on polyacrylonitrile have poor mechanical behavior during cycling, and that gives rise, in particular, to a decrease in the thickness of the electrolyte layer. This creep contributes to premature aging of the cell and shortens the time during which it can be used. This phenomenon is made worse when the cell is required to operate at temperatures in excess of 45.degree. C.
To remedy the problem, proposals have been made to add a reinforcing agent to the electrolyte in the form of an inorganic filler, in particular based on silica. That solution requires about 5% to 10% by mass of additive to be added, representing a large volume because of the low density of the filler, of the order of 36 mg/cm.sup.3.
U.S. Pat. No. 5,631,103 describes an electrolyte having a gelled polymer with a filler constituting more than 50% by weight of the electrolyte. The filler is either an inorganic material or an inert polymer.
Adding such a filler represents a considerable volume. It results in defects of wetting by the plasticizer, and in non-uniformity in filler distribution. This gives rise to dispersion in performance and to lifetime at high temperature that is not improved. In addition, those mixtures have poor pourability which makes them difficult to spread using industrial means.